An important messenger molecule in mammals and other animals. It can be toxic or beneficial, depending upon the amount and where in the body it is released. Initial research into the chemistry of nitric oxide (NO) was motivated by its production in automobile emissions and other combustion processes, which results in photochemical smog and acid rain. In the late 1980s, researchers in immunology, cardiovascular pharmacology, neurobiology, and toxicology discovered that nitric oxide is a crucial physiological messenger molecule. Nitric oxide is now thought to play a role in blood pressure regulation, control of blood clotting, immune defense, digestion, neuronal signaling, the senses of sight and smell, and possibly learning and memory. Underproduction or unregulated overproduction of nitric oxide may also contribute to disease processes such as diabetes, artherosclerosis, stroke, hypertension, carcinogenesis, multiple sclerosis, transplant rejection, damage associated with reperfusion in ischemic (oxygen-deprived) tissues, impotence, septic shock, and long-term depression.